In the field of aerospace, the use of strong and light weight composite materials is increasing. A notorious drawback of these materials is that the process of their failure is rather complex, and damage may develop virtually invisibly from within composite structures, such as aircraft wings and wind turbine blades. To prevent accidents, regular manual inspection of these structures may therefore be required.
It has been suggested to automate the surveillance of mechanically loaded structures by means of an active health monitoring system including a fiber Bragg grating (FBG) sensor that is interrogated by an arrayed wave guide (AWG) based interrogator, which system may infer the existence of various kinds of damage to the monitored structure from changes in the Bragg wavelength. In such health monitoring systems the initial Bragg wavelength of an FBG sensor may typically be situated between the respective center wavelengths of two associated output ports or output channels of the AWG, such that a response provided by the FBG sensor upon interrogation is divided between those ports. When, during use, the Bragg wavelength of the FBG changes due to the fact that it is strained, the power ratio between the two output ports also changes.